Polio, the dreaded paralyzing disease stamped out in the industrialized world, is spreading in Nigeria. And health officials say in some cases, it’s caused by the vaccine used to fight it.

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Experts have long believed epidemics unleashed by a vaccine’s mutated virus wouldn’t last since the vaccine only contains a weakened virus strain — but that assumption is coming under pressure. Some experts now say that once viruses from vaccines start circulating they can become just as dangerous as wild viruses.

“The only difference is that this virus was originally in a vaccine vial,” said Olen Kew, a virologist at the U.S. Centers for Disease Control and Prevention.